Burner safety control apparatus



June 26, 1956 J. A. LOEBER 2,751,972

BURNER SAFETY CONTROL APPARATUS Filed Jan. 28, 1954 2 Sheets-Sheet 1 b MASTER swrrcu ,l98 I55 56 0 I K I 2, Q ELECTRONIC TIMERS 62 ELECTRONIC 46 FLAME 40 32 DETECTOR 4L- INVENTOR. JOHN A. LOEBER ATTORNEY June 26, 1956 LQEBER BURNER SAFETY CONTROL APPARATUS 2 Sheets-Sheet 2 Filed Jan. 28, 1954 G mm mn w u u m it o Wm n ME PS r T w I e 8 .H P. W. w L R m c E a S u g Q 3 3 m INVENTOR.

JOHN A. LOEBER ATTORNEY United States Patent 2,751,972 BURNER SAFETY CGNTROL APPARATUS John A. Loeher, Minneapolis, Minn, assignor to Minne= apolis-Honeyweli Regulator Company, Minneapoiis, Minn, a corporation of Delaware Application January 28, 1954, Serial No. 406,708 Claims. (Ci. 158-28) The present invention is concerned with a control apparatus, and more particularly concerned with a control apparatus for use with a fuel burner unit.

Present day fuel burner units require fast acting control apparatus to insure a safe operation of the burner unit. This requirement has led to the use of detecting devices such as electronic flame detectors. To insure safe operation, it is desirable to check the operation of the flame detector before the burner unit is energized. That is, it is desirable to provide means to check whether or not the flame detector is capable of sensing both the presence and the absence of flame at the fuel burner unit. It is also desirable in an apparatus of this type to provide a safety cutout device, such as a bimetal type safety switch, to be energized in the absence of flame at the fuel burner unit.

It is therefore an object of the present invention to provide a new and improved control apparatus wherein one component of the apparatus is checked before the unit being controlled is energized and wherein a safety cutout device is employed, with the safety cutout device only momentarily energized upon a need for operation of the fuel burner unit being indicated.

It is also desirable in present day control apparatus for fuel burner units to provide a prepurge period during which the combustion chamber is cleared of all combustion vapors and products of combustion. If a single control apparatus is to have utility when used with a great variety of fuel burner unit installations it is necessary to provide for a variable prepurge timing period. However, the addition of this feature to the control apparatus gives safety cutout timing which is not uniform with all prepurge timing periods.

It is therefore a further object of the present invention to provide an improved control apparatus having a prepurge timer and a safety cutout device and means to initially energize the safety cutout device and then deenergize the safety cutout device until the end of the prepurge period.

It is also normal practice to provide a control apparatus which has a flame detector to detect flame at both the pilot and main burners of the fuel burner unit. Upon detecting the presence of pilot flame, the electronic flame detector is effective to cause the main valve to be energized. It has been found that in some cases a bubble of air or water will form in the main fuel line and the flame will be extinguished. The electronic flame detector will sense this to deenergize the main valve. However, there will be fuel on the downstream side of the valve and this fuel in emitting from the main burner will strike the hot refractory surfaces of the burner unit and will be ignited to form a flash of fire which will be detected by the flame detector. The flame detector will then again energize the main valve and more fuel flows to the main burner and an explosion most likely will occur.

It is therefore a further object of the present invention to provide a control apparatus wherein the electronic flame detector is rendered inoperative to detect the presence of flame if the flame fails once the main burner has been energized.

It is a further object of the present invention to provide an improved control apparatus having a flame detector, a safety cutoff device, a prepurge timer, and an ignition timer and having circuitry whereby proper operation of the flame detector is checked before the fuel burner unit being controlled is energized, whereby the safety cutout device is initially momentarily energized to insure that the safety cutout device is operating properly and is re-energized at the end of the prepurge period, and whereby the electronic flame detector is rendered inoperative if flame fails after the end of the ignition timing period.

These and other objects of the present invention will be apparent from the accompanying specification, claims and drawings of which:

Figure l is the schematic showing of the improved con trol apparatus wherein the electronic prepurge and ignition timers and the electronic flame detector are shown in the block form, and

Figure 2 is a schematic showing of the electronic timers and the electronic flame detector shown in block form in Figure 1.

Referring to Figure 1, the reference numeral 10 designates a fuel burner unit having a burner motor 11, a main burner 12, and a pilot burner 13. The fuel flow to main burner 12 is controlled by a main valve 14 while fuel flow to the pilot burner 13 is controlled by a pilot valve 15. The pilot burner is grounded at ground connection 17. Associated with the pilot burner 13 is a flame electrode 18 which is connected to the electronic flame detector by means of a circuit which can be traced through a conductor 19, stationary contact 84 and movable switch blade 79 of a relay 77, conductor 210, and conductor 38 to a signal input terminal 36. This flame electrode 18 is arranged to intersect a flame from the pilot burner 13 and thereby sense a flame emitting from this burner.

Also associated with the pilot burner 13 is an ignition electrode 20 which is connected to an ignition transformer 21. A photoelectric cell 22, which is of the photo emissive type, is positioned to sense the flame at the main burner 12 and is connected by means of conductor 23 and con- P ductor 38 to the signal input terminal 36 of the electronic flame detector, to be later described.

The reference numeral 25 indicates in block form electronic prepurge and ignition timers and an electronic flame detector. Power is furnished to the electronic timers and the electronic flame detector by means of a transformer 26 having a primary winding 27, a first secondary winding 28, a second secondary winding 29 connected to power input terminals 30 and 31 of the electronic timers, and a third secondary winding 32 connected to power input terminals 33 and 34 of the electronic flame detector. The electronic flame detector also has signal input terminals 35 and 36. The signal input terminal 35 is connected to ground at ground connection 37 while the signal input terminal 36 is connected by the circuits above described to the photoelectric cell 22 and the flame rod 18.

Filament power for the electronic flame detector is supplied to terminals 42, 43, and 44. The output of the electronic flame detector is present at output terminals 40 and 41, to which is connected actuating winding 46 of a flame relay 45. Flame relay includes movable switch blades 47, 48, 49, and 51 and stationary contacts 52, 53, 54, and 56. Relay 45 is shown in the deenergized position in Figure 1. Upon energization of the actuating winding 46 of relay 45, movable switch blades 47 and 48 disengage stafionary contacts 52 and 53 respectively and movable switch blades 49, 5t) and 51 engage the stationary contacts 54, 55 and 56 respectively.

The electronic timers, as shown within the block 25,

have filament power input terminals 57, 58, 59, and 60 and have output terminals 61 and 62 associated with the prepurge timer and output terminals 75 and 76 associated with the ignition timer. Connected to the output terminals 61 and 62 is actuating winding 64 of a prepurge timing relay 63. Relay 63 has movable switch blades 65, 66, 6'7, andj68 and stationary contacts .69, 70, 71, 72, 73 and 74. The movable switch blades of relay 63 are shown in their deenergized position in Figure l, and, upon energization of the actuating winding 64, the movable switch blades will move to their energized position wherein switch blades 66 and 67 will disengage contacts 71 and 73 respectively and engage contacts 70 and 72 respectively, and wherein movable switch blades 65 and 68 will engage stationary contacts 69 and 74 respectively.

Connected to the output terminals 75 and 76 is an actuating winding 78 of an ignition timing relay 77. Relay 77 includes movable switch blades 79, 30, 81 and 82 and stationary contacts 83, 84, 85, 6 and 87. The movable switch blades of relay 77 are shown in their deenergized position in Figure 1, and, upon energization of the actuating winding 78, the movable switch blades '80, 81 and 82 will disengage stationary contacts 85, 86 and 87 respectively, and movable switch blade 79 will disengage stationary contacts 84 and move into engagement with stationary contact 83.

The internal structure of the electronic timers and the electronic flame detector is shown in schematic .form in Figure 2 and will be later described.

A proving relay is provided at 88 and includes an actuating winding 89, movable switch blades 90 and 91, and stationary contacts 92 and 93. The actuating winding 89 receives its energizing power from the first secondary winding 28 of transformer 26 in a manner to be later described. The switch blades 90 and 91 of relay 8,8 are shown in Figure l in their deenergized position. Upon energization of the actuating winding 89, movable switch blades 90 and 91 move into engagement with stationary contacts 92 and 93 respectively.

A main control relay is designated by the reference numeral 94 and includes an actuating winding 95, movable switch blades 96, 97, 98, 99, 100, and 101 and stationary contacts 102, 103, 104, 105, 106, and 306. The movable switch blades of relay 94 are shown in Figure l in their deenergized position. Upon energization of actuating winding 95, movable switch blade 100 will disengage stationary contact 106 while movable switch blades 96, 97, 98, 99 and 101 will move into engagement with stationary contacts 102, 103, 104, 105 and 106 respectively.

The energizing power for actuating winding 95 of'relay 94 is supplied by a transformer 107 having a primary winding 108 and a secondary winding 109 having a tap 110. Also in the energizing circuit for the actuating winding 95 of relay 94 is a safety cutout device 111-having a normally closed switch 112, a bimetal element 113, an actuator in the form of a heater 114, and a reset actuator 115. The structure of the safety cutout device 111 is such that the bimetal element 113 warps to the right upon energization of heater 114 and after a predetermined time period of energization of the heater the bimetal element 113 warps sufficiently far to the right to allow the normally closed switch 112 to open. Subsequently, upon cooling of the bimetal element 113, the reset actuator 115 can be depressed to reset the safety cutout device 111 to the condition as shown in Figure 1.

The energizing circuit for primary 103 of transformer 107, to be later described, includes a limit control 116, which may be one or more of a great variety of devices such as a high pressure limit or low water cutoff controls. The energizing circuit may include a manually closed switch 117. When operation of the fuel burner unit is desired, the switch 117 is manually closed to complete its circuit and likewise, when it is no longer necessary for the fuel burner unit 10 to operate, the switch 117 is opened.

Referring now to Figure 2, the electronic prepurge and ignition timers and the electronic flame detectorare shown 4 in schematic form. The power input terminals, filament power input terminals, signal input terminals, and output terminals of the electronic timers and electronic flame detector retain the same reference numeral as used in Figure 1.

Referring specifically to the electronic flame detector, which is shown in the lower half of Figure 2, such as shown in the block diagram 25 of Figure 1, it can be seen that this electronic flame detector utilizes a first electron discharge device 119 and a second electron discharge de' vice 121). A detailed explanation will not be made of the electronic flame detector since this detector is identical to the Richard S. Feigal Patent 2,556,961 to which reference can be made. It suffices to say that in the absence of an input signal at signal input terminals 35 and 36 the electron discharge device 120 is conductive and establishes a voltage across a resistor 121 of the polarity indicated to bias electron discharge device 119 substantially to cutofi. With the discharge device 119 biased substantially to cut off current will not flow between output terminals 40 and 41 and therefore, referring once again to Figure 1, the actuating winding 46 of relay 45 which is connected to these output terminals will not be energized.

When flame is sensed by either flame electrode 18 or photoelectric cell 22, a signal is applied to the signal input terminals 35 and 36 and this signal causes a voltage to be developed across a capacitor 122 of a polarity such as to render electron discharge device 120 substantially nonconductive. Therefore, the voltage across resistor 121 is no longer present and electron discharge device 119 is rendered conductive to thereby energize the actuating winding 46 of relay 45. In this manner the presence or absence of flame at main burner 12 or pilot burner 13 is indicated by the energized or deenergized state respectively of the actuating winding 46 of relay 45.

From the above explanation it can be seen that with prolonged operation of the electronic flame detector the electron discharge devices 119 and 120 may deteriorate to a point where the conducting qualities of the discharge devices are no longer of the quality necessary to energize the actuating winding 46 or to produce a voltage across the resistor 121. It therefore becomes desirable to provide a means to check the proper operation of the electronic flame detector. That is, to check the fact that electronic discharge device 119 is capable of conducting sufliciently to energize the relay winding 46 and that the electron discharge device 120 is capable of conducting sufficiently to produce a voltage across resistor 121 to bias electron discharge device 119 substantially to cutoff. The manner in which this is done will be described later.

Referring now specifically to the electronic timers of Figure 2, a prepurge timer 130 including an electron discharge device 125 and an ignition timer 131 including an electron discharge device 126 are shown. A direct current power supply 127 is shown connected to the secondary winding 29 of relay 26. The direct current power source 127 includes a capacitor 123 which is charged to the polarity indicated.

A prepurge timing selector switch 129 is provided and is effective to change the timing period of the prepurgc timer. An ignition timing selector switch 154- is pro vided and is effective to change the timing period of the ignition timer.

Operation of the prepurge timer can be explained by first assuming that the selector switch 129 is open, as is the position shown in Figure 2, and further assuming that power is initially supplied to filament power input terminals 57 and 58. A predetermined time after power is applied to the terminals 57 and 58 the filament of the electron discharge device 125 will be heated to an operative temperature and the discharge device 125 will conduct current. This conducting circuit can be traced from a cathode 132 to an anode 133 through a conductor 134 to the positive terminal of capacitor 128 and from the ne at te m n of c pa 1 3 th g a coa uewr 135, conductor 136 to Output terminal 62. Upon reference to Figure 1, it can be seen that output terminal 62 connects to actuating winding 64 of relay 63 and back to output terminal 61. From output terminal 61 the circuit is completed through a conductor 137, conductor 138, to cathode 132. The timing function performed by the prepurge timer 130 with selector switch 129 open is determined by the heating time of the filament of the electron discharge device 125. This, for example, can be five seconds.

If the selector switch 129 is closed, the timing period is extended, for example, to 30 seconds. This can be explained by assuming that power is again initially applied to filament power input terminals 57 and 58. With the selector switch 129 closed, a capacitor 139 is connected in series with a resistor 146 and this series combination is connected in parallel to the winding 64 of relay 63. Therefore, in the above traced conducting circuit the voltage which is developed across winding 64 to tend to energize the winding is also applied across the series combination of the resistor 140 and capacitor 139. As the filament of discharge device 125 heats, the current flowing in the above traced conductive circuit gradually increases. This current in flowing through relay coil 64 produces a voltage across this coil. In parallel with coil 64 is a series circuit consisting of resistor 140 and capacitor 139. Capacitor 139 is in the uncharged state. The initial voltage developed across winding 64 is therefore likewise developed across resistor 140. Resistor 140 is connected directly between cathode 132 and control electrode 141. The voltage developed across this resistor causes the discharge device 125 to be biased in a manner to oppose this increasing current flow. This operation continues as the charge on capacitor 139 gradually builds up. When capacitor 139 is sutficiently charged, the voltage across resistor 140 will not he sufficient to hold the conducting state of discharge device 125 below that value necessary to energize actuating winding 64 of relay 63. Relay 63 then becomes energized and, as stated above, this can be made to occur normally 30 seconds after the power is supplied to filament power input terminals 57 and 58.

The ignition timer 131 makes use of an electronic timer which is substantially identical to the electronic prepurge timer 130. In the ignition timer 131 the timing interval is initiated by applying power to filament power input terminals 59 and 61). However, in this case the resistor 141) of the prepurge timer 130 is replaced by the resistors 150, 151, and 152. The ignition timing selector switch in one position places the resistor 150 in parallel with the resistor 151 and in series with resistor 152. When the ignition timing selector switch is closed the resistors 151) and 151 are shorted out so that the RC time constant of the circuit including these three resistors and a capacitor 153 is changed to thereby change the timing of the ignition timer 131. The conducting circuit for the electron discharge device 126 includes the output terminals 75 and 76 to which, referring again to Figure 1, is connected the winding 78 of relay 77. With the ignition timing selector switch in the open position, such as shown in Figure 1, an ignition timing period for example of approximately 60 seconds may be obtained. In other words, the winding 78 of relay 77 will be energized approximately 60 seconds after filament power is supplied to filament power input terminals 59 and 60. With the ignition timing selector switch closed an ignition timing period for example of approximately seconds may be obtained.

Operation The operation of the improved control apparatus as it controls the operation of the fuel burner unit 10 will now be described. Referring once again to Figure 1, power is supplied to the improved burner control apparatus and burner unit 10 by means of power line conductors 155 and 156 and a master switch With the apparatus as shown in Figure 1, power is not applied to the power line conductors 155 and 156 since the master switch is open. it will first be assumed that the master switch is closed and that at this time the switch 117 is not closed. With power applied to the power line conductors and without switch 117 closed the only primary energized is the primary 27 of transformer 26. This energizes secondaries 28, 29 and 32.

Energization of secondary 28 energizes the filament of the electron discharge device 119 associated with the electronic flame detector. This can be seen by tracing {a circuit from the upper terminal of secondary 28 through conductor 157, conductor 158, to filament power input terminal 43 of the electronic flame detector. Upon reference to Figure 2, the circuit can be traced to the filament of the electron discharge device 119, and to filament power input terminal 42. Referring once again to Figure 1, the circuit can be traced from conductor 160 to conductor 161, conductor 162, and conductor 163 to the lower terminal of secondary 28.

With the secondary 32 of transformer 26 energized, power is applied to electron discharge device 119 in the electronic flame detector and after a predetermined time period necessary for the fiiament of this electron discharge device to become operative relay winding 46 connected to the output terminals 41; and 41 is energized. At this time the electron discharge device 12 3- associated with the electronic flame detector is not operative for the reason that its filament circuit is open. One energizing circuit for this filament will be pointed out later.

Energization of actuating winding 4-6 of flame relay 45 causes movable switch blades 47, 4-8, 49, 50 and 51 to move to their energized position. Energization of the flame relay 45 causes the proving relay 38 to be energized. The energizing circuit for the proving relay 38 can be seen by tracing a circuit from the upper terminal of secondary 28 through a conductor 16 i, actuating winding 89 of relay 33, conductor 165, conductor 166, stationary contact 54 and movable switch blade 49 of relay 45, conductor 161, conductor 162, and conductor 163 to the bottom terminal of secondary 28. A holding circuit has been provided for the actuating winding 89 of relay 88 and includes a resistor 167. The level of energization furnished the actuating winding 89 by means of the holding circuit including resistor 167 is of a comparatively low level and is incapable of initially actuating the relay. However, once the relay is energized, the relay will be held in the energized position by means of this holding circuit.

With energization of relay S8, movable switch blades 9t) and 91 move to their energized position. The movable switch blade 9% in its energized position is eflfective to complete a portion of the initial energizing circuit for the main control relay 94, to be later described. Switch blade 91 in its energized position completes an energizing circuit for the filament of the electron discharge device 120 in the electronic flame detector. This energizing circuit can be seen by tracing a circuit from the upper terminal of the secondary 23 through conductor 157, conduo-tor 15:3, to filament power input terminal 143 of the electronic flame detector. Upon reference to Figure 2 the circuit can be traced through the filament associated with the electron discharge device 129 to filament power input terminal 44. Referring once again to Figure l, the circuit is completed from conductor 168 to conductor 169, stationary contact 1126 and movable switch blade of relay 94, conductor 1'79, conductor 171, stationary contact 93 and movable switch blade 91 of relay 88, conductor 172, conductor 162, and conductor 163 to the lower terminal of the secondary 28.

Energization of the filament of electron discharge device in the electronic flame detector causes the discharge device 129 to become conductive and establish the biasing voltage across resistor 121, such as indicated in Figure 2. Therefore, the electron discharge device 119 is rendered nonconductive and the flame relay 4-5 is deenergized.

The control apparatus is now in the standby condition and will remain in this condition until the switch liii'l" is depressed indicating that there is a need for operation of the fuel burner unit 1%. In reviewing the operation of the control apparatus up to this point it will be noted that the flame relay 45 has been energized. and subsequently deenergized and the proving relay been energized. In first energizing the relay 45, the proper operation of the electronic disch irge device 119 was chacued to insure that this discharge device was capable of conducting sufficiently to energize flame relay 45. Subsequently, when the proving relay 33 became energized, the electron discharge device 12% was rendered conductive to check the proper operation of this electron discharge device to insure that it was capable of establishing proper bias voltage across the resistor 121 to cause flame relay 45 to drop out.

It will now be assumed that the switch 1'17 is manually closed and thereby call for operation of the burner unit 10. It will be recognized that an automatic type of control such as the thermostat can be substituted for the switch 117 shown in Figure 1.

Closing the switch 117 completes an energizing circuit for the primary 163 of the transformer E 3 This energizing circuit can be traced from power line conductor 156 through conductor 17.5, limit control 15.6, switch 117, conductor 17 i, movable switch blade 3% and stationary contact 35 of relay 77, stationary contact 73 and movable switch blade 67 of relay 63, conductor 175, conductor 176, and primary 1% to the power line conductor 1.55. it will be noted from this above traced circuit that in order for this circuit to be completed the electronic prepurge timer 130 and the electronic ignition timer 131, shown in Figure 2, must be inoperative so that the associated relay 63 and 77 respectively will be in the deenergizcd position.

Energization of primary Ellis energizes secondary 1G9 and thereby energizes actuating winding d5 of relay 94. This energizing circuit can be seen by tracing a circuit from the upper terminal of secondary i through the normally closed switch 112 of safety cutout device 111, conductor 1 7, winding $5 of relay 9 E, conductor 3%, movable switch blade 48 and stationary contact 53 of relay 45, conductor 1%, conductor 13%, stationary contact 92 and movable switch blade 9t; of relay 8%, conductor 179, heater 114, and conductor 2% to the lower terminal of secondary 1&9. It can be seen from this above traced circuit that it is necessary in order for this circuit to be completed that the proving relay 3% be in the energized position and that the flame relay 45 be in the deenergized position. in this manner it is insured that the main control relay 94 will not be energized unless the electronic flame detector and proving relay 3% have functioned properly, as above described.

Energization of main control relay 94 causes the associated movable switch blades to move to their energize positions. Movable switch blades as and 97 in moving into engagem nt with stationary contacts Hi2 and 03 respectively complete a holding circuit for the main control relay 94 which can be traced from the upper terminal of secondary 139 through safety cutout device 111, conductor 177, winding 95, stationary contact Hi2. and movable switch blade 96, conductor 193, stationary contact 1% and movable switch blade Q7, conductor and conductor 195 to the tap lid of secondar m9.

Movable switch blade 99 in moving into engagement with stationary contact 1 55 completes a circuit which shunts a portion of the energizing circuit for the primary 1% of transformer 1'97. it will be remembered that the energizing circuit for the primary 1% included the series connected movable switch blade 38 of relay '77 and movable switch blade 67 of relay 63. This portion of the circuit wi l be broken subsequently as the prepurge timer and ignition tiiner 131 are put into operation and it is therefore necessary for this portion of the circuit to be shunted as it is by movable switch blade 9t) and stationary contact 1H5.

Upon energization of the main control relay 94, movable switch blade ltil moves into engagement with stationary contact 3%. An energizing circuit can now be traced for burner motor ll, from power line conductor 156 through conductor 17.3, conductor 1%, stationary Contact see and movable switch blade Hill, conductor or 2%, burner motor 3.1, and conductor 201 to power line conductor 155. Operation of burner motor 11 is thereby initiated to purge the combustion chamber of the fuel burner unit 10, not shown, of combustible vapors and products of combustion.

As soon as secondary ltlh of transformer 107 is energized the filament of electron discharge device 125 in the prepurge timer 13b is energized. The energizing circuit for this filament can be traced from the tap ill of secondary 1 39 through conductor 195, and conductor 1% to filament power input terminal 57. Upon reference to Figure 2, the circuit can be traced through the filament of electron discharge device 125 to filament power input terminal 53. Referring once again to Figure 1, the circuit is completed through conductor 1 .97, conductor 177,

safety cutout device ill to the upper terminal of secondary 3. 39. The prepurge timing period is thus initiated and with the prepurge timing selector switch in the open position, such as it is shown in Figure 2, relay 63 will be energized in approximately five seconds.

Energizatiou of relay 94 also causes movable switch blade 1% to disengage stationary contact res. It will be observed that movable switch blade Mill is in the above traced energizing circuit for the filament of electron discharge device 129, which discharge device is associated with the electronic flame detector as shown in Figure 2. However, at the same time that this circuit is broken a circuit is completed whereby secondary 1 9 is utilized to maintain the filament of electron discharge device 126 cuergized. This can be seen by tracing a circuit from the lower terminal of secondary res through conductor 192, conductor 164, conductor 157, conductor 158, conductor 159 to filament power input terminal 4-3. Referring to Figure 2 the circuit is continued through the filament of electron discharge device 120 to filament power input teri inal 4d. Referring again to Figurel, the circuit is completed through conductor 163, conductor 2oz, stationary contact 183 and movable switch blade 97 of relay 9%, conductor 194, and conductor to the tap 11b of secondary 169.

Energization of the main control relay 94 also causes the movable switch blade 95 to move into engagement with stationary contact TAM. This completes a circuit which shunts the winding d9 of proving relay 8% to thereby cause this relay to be decnergized. This shunt circuit can be seen by tracing a circuit from the right-hand end of winding 89 through conductor 165, conductor 203, stationary contact 194' and movable switch blade 89, con ductor 2M, stationary contact 71 and movable switch blade d6 of relay 63, conductor 295, conductor 157, and conductor 1&4 to the left-hand end of winding 8%. It can be seen that this above traced shunt circuit includes the movable switch blade 66 of relay 63 and that this shunt circuit will be broken at the end of the prepurge period when relay 63 is energized.

To explain the utility in requiring that the proving relay 88 be deenergized at this time it is necessary to again refer to the energizing circuit for the main control relay 94. It will be remembered that this energizing circuit included the heater of safety cutout device 111. It is desirable to require that the initial energization of the main control relay 4 include this heater since it would not be desirable to allow relay 94 to be energized if this heater were open circuited due to some fault within the heater. However, the safety cutout device 111 is constructed to open its normally closed contact 112 after a predetermined time period of energization of the heater 114. When a variable prepurge timer, such as timer 130, is used it is desirable to delay energization of the actuating heater 114 until the control apparatus is in a condition to attempt to light the pilot burner 13. It is not desirable to have the heater 11.4 energized during the variable prepurge period. Therefore, deenergization of the proving reia S3 breaks the initial energizing circuit for the main control relay 94 and thereby deenergizes actuating heater 114. It will be remembered that the main control relay 94 is maintained energized through a circuit which includes the movable switch blade 96 and movable switch blade 97 of relay 94.

The control apparatus is now in a prepurge mode of operation where the burner motor 11 is energized to purge the combustion chamber. The apparatus will continue in this mode of operation until the end of the prepurge time, which is variable, and is set to a value depending upon the particular fuel burner unit installation being controlled by the improved control apparatus.

The end of the prepurge time is indicated by energization of the relay 63 by the prepurge timer 130. Energization of relay 63 causes movable switch blade 68 to engage stationary contact 74. This completes a circuit which shunts movable switch blade 101 and stationary contact 386 of the main control relay 94. The function of this is to provide for a postpurge period, to be later described.

Energizatio-n of relay 63 also causes movable switch blade 67 to disengage stationary contact 73 and move into engagement with stationary contact 72. Stationary contact 73 is in the energizing circuit for the primary 168, as above described, however a holding circuit for primary 1% is completed at this time through movable switch blade 99 and stationary contact 1ti5 of relay 94.

When movable switch blade 67 engages stationary contact 72, an energizing circuit is completed for the pilot valve 15 and the ignition transformer 21. This energizing circuit can be seen by tracing a circuit from power line conductor 156, through conductor 173, limit control 116, start-stop actuator 117, movable switch blade 99 and sta tionary contact 165 of relay 94, conductor 175, movable switch blade 67 and stationary contact 72 of relay 63, conductor 267, movable switch blade 81 and stationary contact 86 of relay 77, conductor 208 to connected pilot valve 15 and movable switch blade 87 and stationary contact 87 through conductor 391 to ignition transformer 21, to the power line conductor 155. A flame normally now appears at the pilot burner 13.

At the same time that the pilot valve 15 and ignition transformer 21 are energized, the heater 114 of the safety cutout device 1 11 is again energized through a circuit which can be traced from the lower terminal of the secondary 109 through conductor 208, heater 114, conductor 179, stationary contact 69 and movable switch blade 65 of relay 63, conductor 19d, stationary contact 53 and movable switch blade 48 of flame relay 45, conductor 209, stationary contact 102 and movable switch blade 96 and stationary contact 193 and movable switch blade 97 of relay 94, conductor 194, and conductor 195 to the tap 116 of secondary 109. Therefore, unless flame is established at the pilot burner 13 and detected by the electronic flame detector to energize the flame relay 45 and thereby break the above traced circuit, the safety cutout device will be operative at the end of its timing period to break the energizing circuit to the main control relay 94.

Normally however flame is established at the pilot burner 13 and is detected by the electronic flame detector to energize the flame relay 45. As above mentioned, energization of the flame relay 45 breaks the above traced energizing circuit to the heater 114 of the safety cutout device 111. Also, energization of the flame relay 45 completes a holding circuit for the pilot valve 15 to maintain this pilot valve 15 energized when the relay 77 is energized at the end of the ignition timing period. This can be seen by observing that the initial energizing circuit for the pilot valve 15 includes the movable switch blade 81 engaging stationary contact 86 of relay 77. At the end of the ignition timing period, to be subsequently described, this circuit is broken and the pilot valve 15 is maintained energized through the movable switch blade 50 and stationary contact 55 of flame relay 45, which movable switch blade and stationary contact shunt the above mentioned movable switch blade 81 and stationary contact 86 of relay 77.

Energization of the flame relay 45 also energizes the main burner valve 14 through a circuit which can be traced from the power line conductor 156 through conductor 173, limit control 116, start-stop actuator 117, movable switch blade 99 and stationary contact of main control relay 94, conductor 175, movable switch blade 67 and stationary contact 72 of relay 63, conductor 2G7, stationary contact 55 and movable switch blade 50 of flame relay 45, conductor 212, stationary contact 56 and movable switch blade 51 of flame relay 45, conductor 213, main valve 14, and conductor 201 to power line conductor 155. A flame is normally now established at the main burner 12.

Energization of the flame relay 45 also causes movable switch blade 49 to engage stationary contact 54. This again completes the initial energizing circuit for the proving relay 88 and since the circuit which shunted the winding 89 of the proving relay is now broken at movable switch blade 66 and stationary contact 71 of the relay 63, the proving relay S8 is again energized.

Upon energization of the relay 63 at the end of the prepurge period and upon re-energization of the proving relay 88, an energizing circuit is completed for the filament of electron discharge device 126 of the ignition timer 131. This circuit can be seen by tracing a circuit from the upper terminal of secondary 28 through conductor 157, conductor 205, movable switch blade 66 and stationary contact 70 of relay 63, and conductor 214, to filament power input terminal 69. Referring to Figure 2, the circuit continues through the filament of electron discharge device 126 to filament power input terminal 59. Referring again to Figure l, the circuit is completed through conductor 215, conductor 171, stationary contact 93 and movable switch blade 91 of proving relay 88, conductor 172, conductor 162, and conductor 163 to the lower terminal of secondary 28. Therefore, at this time the ignition timing period is initiated and at a predetermined time thereafter the relay 77 will be energized. With the ignition selector switch 154 in the open circuit position, such as shown in Figure 2, relay 77 will be energized approximately 60 seconds after energization of the filament of electron discharge device 126.

After the 60 second ignition timing period has elapsed relay 77 will be energized to actuate its associated movable switch blades into their energized position. When movable switch blade 81 disengages stationary contact 86 the power which is supplied to the pilot valve 15 and main valve 14 is dependent upon the electronic flame detector continuously detecting flame to maintain movable switch blade 50 engaged with stationary contact 55. When movable switch blade 81 disengages stationary contact 87 the ignition transformer 21 is deenergized.

Energization of relay 77 causes movable switch blade '79 to disengage stationary contact 84 and engage stationary contact 83. When stationary contact 84 is disengaged, the fiame electrode 18 is disconnected from the electronic flame detector and in order for the electronic flame detector to continuously detect flame the photoelectric cell 22 must sense the presence of flame at the main burner 12.

When switch blade 79 engages stationary contact 83 a safety circuit is conditioned to be completed upon sub- 11 sequent flame failure. to render the electronic flame detector inoperative, as will be described later.

The control apparatus is now in the running mode of operation. That is, the ignition transformer 21 is deenergized, the pilot valve 15 is energized, the main valve 14 is energized, and the burner motor 11 is energized. The safety cutout device heater 114 is deenergized, prepurge timer 136 and ignition timer 131 are maintaining relays 63 and 77 in the energized condition, and proving relay 88 is energized.

in order to explain the above mentioned safety feature it is necessary to assume that at this time there is a flame failure at the main burner 12. It has been found that this flame failure may be caused by a bubble of air or water forming in the fuel line. This causes the flame to momentarily be extinguished at the burner 12 and the electronic flame detector senses this to deenergize the flame relay 45. This deenergizes the main valve and the pilot valve to stop the flow of fuel to the combustion chamber. However, there is fuel on the downstream side of the main valve and this fuel is emitted from the main burner 12 and strikes the hot refractory surface of the combustion chamber to be ignited with a flash of light. This flash of light is detected by the electronic flame detector and the flame relay i is again energized. This aga n completes an energizing circuit to the main valve 14 and fuel is once again emitted from the main burner 12 and strikes the hot refractory surface. In this instance quite a large quantity of fuel is released into the combustion chamber and an explosion may occur. vent this, a safety circuit has been provided at the signal input of. the electronic flame detector. If at this time the flame relay 45 becomes deenergized, a circuit can be traced which shunts the signal input terminals 35 and 36. This circuit can be traced from the signal input terminal 36 through conductor 38, conductor 210, movable switch blade 79 and stationary contact 33 of relay '77, stationary contact 52 and movable switch blade 47 of flame relay 45, and conductor 211 to the signal input terminal 35.

With the signal input terminals shunted the momentary flash of light which occurs when the fuel on the downstream side of the valve strikes the refractory surface will not cause the electronic flame detector to again pull in and again energize the main valve 14-.

Normally'however, the control apparatus and the burner unit continue to operate in their running condition until the switch 117 is manually opened. The depressing of this stop button deenergizes the main control relay 94, the valves 14- and 15, and the filaments of discharge devices 125 and 126 of prepurge timer 130 and the ignition timer 131 respectively. However, due to the fact that the filaments of these electron discharge devices must cool to a certain extent before the electron discharge devices become nonconductive the relays 63 and 77 remain energized for a given period depending upon the cooling rate of the electron discharge devices. As was above mentioned, the burner motor 11 will remain energized to provide a postpurge period so long as the movable switch blade 63 engages stationary contact 74 of relay 63.

Since many modifications of the present invention will be apparent to. those skilled in the art, it is intended that my invention be limited only by the scope of the appended claims.

I claim as my invention:

1. A burner control system comprising, a fuel burner unit comprising an electrically energizablc fuel feeding means and blower, time delayed safety cutout means having an electrically energizable actuator and a switch,

means normally causing energization of said safety cutout means actuator upon the absence of flame at the fuel burner unit, first electrically operable means for energizing said blower, means to initially energize said first means including the actuator and switch of said safety To precutout means, holding means including said safety cutout means switch to maintain said first means energized, means controlled by said first means to deenergize said safety cutout means actuator upon energization of said first means, prepurge timing means to delay energization of said fuel valve for a time period after energization of said blower, manually selectable means to vary the time delay period of said prepurge timing means, and means rendered operable by said prepurge timing means to energize the actuator of said safety cutout means 2t the end of said time delay period.

2. Control apparatus for use with a fuel burner unit having an electrically energizable fuel valve and blower motor, comprising, a safety cutout device having an electrically energizable actuator and a normally closed switch which is actuated to open position after predetermined energization of said actuator, a control relay having an electrically energizable actuator and switch means, a source of power, energizing circuit means including the switch and actuator of said safety cutout device connecting said control relay actuator to said source of power to thereby energize said control relay actuator, holding circuit means including the switch of said safety cutout means and the switch means of said control relay connecting said control relay actuator to said source of power, means to open said energizing circuit and thereby deenergize the actuator of said safety cutout device upon said control relay being energized, circuit means including the switch means of said control relay adapted to cause energization of the blower motor and fuel valve, a prepurge timer to delay energization of the fuel valve, manually operable means to select a prepurge timing interval, and means rendered operable by said prepurge timer to again energize the actuator of said safety switch at the end of said prepurge timing interval.

3. Burner control system comprising, a fuel burner unit having an electrically energizable fuel supplying means and blower motor, control means to initiate operation of said blower motor, variable time delay purge timing means connected to delay energization of said fuel supplying means to provide for a period of operation of the blower motor before fuel is supplied, flame detecting means for detecting flame at said fuel burner unit, time delayed safety cutout means having an electrically energizable actuator and a switch, circuit means including said actuator and means controlled by said flame detector to normally cause said actuator to be en ergized in the absence of flame, circuit means for said control means including said actuator and switch whereby said control means is inoperative to initiate operation of said blower motor unless said actuator has electrical continuity, means including said switch to maintain said control means operative independent of said actuator once said control means is rendered operative, and means rendered operable by said purge timing means to energize said actuator at the end of said time delay period to thereby start the timing of said safety cutout means concurrent with the supplying of fuel.

4. Burner control system comprising, a fuel burner unit having an electrically energizable fuel feeding means and blower, electrically energizable control means for energizing said blower, prepurge timing means for energizing said fuel feeding means a time period after energization of said blower, flame detecting means for detecting flame at said fuel burner unit, flame simulating means for initially simulating flame at said fuel burner unit to normally cause said flame detecting means initially to detect flame, checking means rendered operative upon said flame detectin means initially detecting flame and effective upon being rendered operative to render said flame simulating means inoperative, time delayed safety cutout means having an electrically energizable actuator and switch, energizing circuit means for energizing said control means including the actuator of said safety cutout means'and controlled by said safety cutout means switch, said checking means, and said flame detecting means, said circuit means being completed when the actuator of said safety cutout means has electrical continuity, when said checking means is operative, and when said flame detecting means detects the absence of flame at the fuel burner unit, holding circuit means for said control means, means effective upon energization of said control means to open circuit said energizing circuit means, and means to close said energizing circuit means concurrent with energization of said fuel feeding means by said prepurge timing means.

5. A burner control system comprising, a fuel burner unit having an electrically energizable fuel valve and a blower motor, first electrically energizable means for bringing into operation said blower motor, electrically energizable prepurge timer means rendered operative by said first means and effective after a predetermined time to energize said fuel valve, safety cutout means having an electrically energizable actuator and a switch connected in controlling relation to said first means, electrically operable control means whereby said first means is energized only if electrical continuity exists in said safety cutout means actuator, and further electrically operable control means to effect operative energization of said safety cutout means actuator when said fuel valve is energized.

6. Control apparatus for use with condition producing means, comprising: a main control relay having an electrically energizable actuator and switch means, a proving relay having an electrically energizable actuator and switch means, electrical condition responsive means including condition sensing means and a condition sensing relay having an electrically energizable actuator and switch means, said condition sensing relay actuator being energized upon said sensing means sensing a condition produced by the condition producing means, a source of power, circuit means connecting said condition responsive means to said source of power to cause the actuator of said condition sensing relay to be energized, circuit means controlled by said condition sensing relay switch means connecting the actuator of said proving relay to said source of power to energize said proving relay actuator upon said condition sensing relay actuator being energized, a self-holding circuit for said proving relay actuator to maintain said proving relay actuator energized once said condition sensing relay actuator is energized, means controlled by the said proving relay switch means to cause the actuator of said condition sensing relay to be subsequently de-energized upon said proving relay actuator being energized, time delayed safety cutout means having an electrically operable actuator and switch means actuated thereby, energizing circuit means controlled by the switch means of said proving and condition sensing relays and including said safety cutout means actuator connecting said main control relay actuator to said source of power upon said proving relay actuator being energized and said condition sensing relay actuator being de-energized, a holding circuit for said main control relay actuator including said safety cutout switch means, means controlled by said main control relay switch means to cause energization of the condition changing means, and circuit means controlled by the said main control relay switch means shunting the actuator of said proving relay to cause said proving relay actuator to be de-energized to thereby open said energizing circuit means and prevent actuation of said time delayed safety cutout switch means.

7. Control apparatus for use with condition producing means, comprising: a source of voltage, condition responsive means having condition sensing means and an electrically operable actuator and switch means, said sensing means being sensitive to the condition produced by the condition producing means, circuit means connecting said condition sensing means actuator to said source of voltage to cause said condition sensing switch means to be actuated from a first to a second condition, proving means having an electrically operable actuator and switch means, circuit means controlled by said condition sensing switch means in said second condition connecting said proving means actuator to said source of voltage to cause said proving switch means to be actuated from a first to a second condition, means controlled by said proving switch means in said second condition to cause said condition sensing switch means to return to said first condition, holding circuit means to maintain said proving means actuator operatively connected to said source of voltage, main control means having an electrically operable actuator and switch means, time delayed safety cutout means having an electrically operable actuator and switch means actuated thereby, circuit means controlled by said condition sensing switch means in said first condition and said proving switch means in said second condition and including the actuator of said safety cutout device connecting said main control means actuator to said source of voltage to thereby cause said main control switch means to be actuated from a first to a second condition, holding circuit means for said main concontrol means actuator including said safety cutout switch means, means controlled by said main control switch means arranged to energize the condition changing means, and circuit means controlled by said main control switch means in said second condition rendering said proving means inoperative to thereby cause said proving switch means to return to said first condition.

8. Burner control apparatus for use with a fuel burner comprising: flame detection means including a flame relay having an electrically energizable actuator and switch means actuated thereby, said flame relay actuator being energized upon said flame detection means detecting flame at the fuel burner, power input terminals adapted to be connected to a source of power, circuit means connecting said flame detection means to said terminals, flame simulating means normally causing said flame relay actuator to be energized independent of actual flame at the fuel burner upon initial connection of said terminals to a source of power, a proving relay having an electrically energizable actuator and switch means actuated thereby, circuit means controlled by said flame relay switch means when said flame relay actuator is energized connecting said proving relay actuator to said terminals to thereby energize said proving relay actuator, holding circuit means for said proving relay actuator, means controlled by said providing relay switch means to render said flame simulating means inoperative and thereby cause said flame relay actuator to be de-energized in response to the absence of flame at the fuel burner, delayed action safety cutout means having an electrically energizable actuator and a switch, a main control relay having an electrically energizable actuator and switch means actuated thereby and adapted to energize the fuel burner, energizing circuit means controlled by said flame relay switch means and said proving relay switch means when said flame relay actuator is de-energized and said proving relay actuator is energized and including said safety cutout means actuator and switch connecting said main control relay actuator to said terminals, holding circuit means for said main control relay actuator including said safety cutout means switch, and circuit means controlled by said main control relay switch means to de-energize said proving relay actuator and thereby de-energize said energizing circuit means and said safety cutout means actuator.

9. Burner control apparatus for use with a fuel burner having an electrically energizable blower motor and fuel valve, comprising; time delayed safety cutout means having an electrically energizable actuator and a switch, a flame detector including flame sensing means adapted to sense the presence of flame at the fuel burner and including switch means actuated from a first to a second condition upon the detecting of flame, power input terminals, means responsive to the initial application of power to said terminals to cause said flame detector switch means to be actuated from said first to said second condition independent of actual flame at the fuel burner and also responsive to said flame detector switch means assuming said second condition to cause the switch means of said flame detector to once again assume said first condition in response to the absence of flame at the fuel burner, main control switch means having an electrically energizable actuator adapted to cause energization of the blower motor upon said main control switch means being actuated from a first to a second condition, a prepurge timer having an electrically energizable actuator and switch means actuated thereby and adapted to cause energization of the fuel valve a predetermined time after said timer actuator is energized, circuit means controlled by said main control switch means in said second condition connecting said timer actuator to said terminals to thereby energize said timer actuator, energizing circuit means including the actuator and switch of said safety cutout means and adapted to be completed upon said flame detector switch means being actuated from said first to said second and back to said first condition to connect the actuator of said main control switch means to said terminals to thereby cause the actuator of said main control switch means to be energized, holding circuit means for the actuator of said main control switch means including said safety cutout means switch, means whereby energization of the actuator of said main control switch means causes said energizing circuit means to be broken to thereby de-energize the actuator of said safety cutout means, and means controlled by the switch means of said prepurge timer to energize the actuator of said cutout means when the fuel valve is energized.

l0. Burner control system comprising, an electrically energizable fuel burner unit, power input terminals, a first relay having an electrically energizable actuator and switch means connecting said fuel burner unit to said power input terminals when said first relay actuator is energized, a flame detector for detecting the presence of flame at said fuel burner unit and including a second relay having switch means and an electrically energizable actuator which is energized in the presence of flame, flame simulating means to cause said second relay actuator to be energized following initial application of power to said power input terminals, 21 third relay having an electrically energizable actuator and switch means, means energizing said third relay actuator upon said second relay actuator being energized, holding means to maintain said third relay actuator energized independently of said second relay, means controlled by said third relay switch means to render said flame simulating means inoperative to thereby cause said second. relay actuator to be deenergized, time delayed safety cutout means having an electrically energizable actuator and a switch, circuit means for energizing said first relay actuator including the actuator and switch of said safety cutout means and controlled by said second and third relay switch means, said last named circuit means being completed when the actuator of said safety cutout means has electrical continuity and when said second relay actuator is de-energized and said'third relay actuator is energized, holding circuit means for said first relay actuator to maintain said first relay actuator energized, said holding circuit means including said safety cutout switch, and means controlled by said first relay switch means to de-energize said third relay actuator upon said first relay actuator being energized.

ll. Control apparatus ror use with a fuel burner unit having a pilot and a main burner with electrically operable valves associated therewith, and having electrically operable pilot ignition means, the apparatus comprising;

an electrical flame detector having electrical circuitry andincluding a flame sensing element adapted to be positioned to sense flame at the pilot and main burner, a flame relay having an electrically energizable actuator connected to said flame detector to be energized upon said flame sensing element sensing a flame, switch means controlled by said flame relay actuator; an ignition timer having an electrically operable actuator and switch means controlled thereby; control means for energizing the pilot valve and ignition means and for energizing said ignition timer actuator to thereby cause a pilot flame to be established at the pilot burner; means controlled by said flame relay switch means upon said flame detector detecting a pilot flame to cause energization of the main valve to thereby cause a flame to be established at the main burner; means controlled by said ignition timing switch means to de-energize said ignition means a timed period after energization of said ignition timer actuator; and means controlled by said flame relay switch means and said ignition timer switch means to alter the electrical circuitry of said electrical flame detector to render said flame detector inoperative to sense the presence or absence of flame in the event that said flame sensing means senses the absence of flame after said timed period after which said ignition timer switch means has de-energized the ignition means.

12. Control apparatus for use with a fuel burner unit having a pilot and a main burner, an electrically energizable main burner valve, and having electrically energizable ignition means for the pilot burner, comprising: an electronic flame detector having switching means controlled thereby and having signal input terminals, flame sensing means adapted to be located in the proximity of the pilot and main burners to sense flame at the burners, circuit means connecting said flame sensing means to said signal input terminals, a timer having an electrically energizable actuator and switch means actuated thereby from a first to a second condition a time period after initial energization of said timer actuator, circuit means for energizing the ignition means including said timer switching means in said first condition, circuit means including said flame detector switching means for energizing the main burner valve when flame is detected at the pilot burner, means for energizing said timer actuator upon energization of the ignition means, and shunt circuit means connecting said input terminals when said flame detector switching means is in said first condition and said timer switch means is in said second condition to thereby shunt said flame sensing means and render said flame detector inoperative to detect flame if a flame failure is sensed by said flame sensing means after said time period.

13. Control system comprising, a fuel burner unit having a pilot and a main burner with electrically operable valves associated therewith, electrically operable pilot ignition means, and a blower motor; an electrical flame detector for detecting flame at said pilot and main burners, a prepurge timer having an electrically energizable actuator and switch means controlled thereby; an ignition timer having an electrically energizable actuator and switch means controlled thereby; means for energizing said blower motor and said prepurge timer actuator upon a need for operation of said fuel burner unit; means controlled by said prepurge timer switch means for energizing said pilot burner valve, said ignition timer actuator and said ignition means a time period after energization of said blower motor; means controlled by said flame detector for energizing said main burner valve upon pilot flame being established; means controlled by said ignition timer switch means for de-energizing said ignition means a time period after said ignition means is energized; and means controlled by said flame detector and said ignition timer switch means to render said flame detector inoperative to indicate the presence of flame upon flame failure after said ignition means has been de-energized.

14. Burner control system comprising, a fuel burner unit having electrical ignition means, a pilot valve and burner, a main valve and burner and having a blower; control means having an electrically operable actuator and having switch means actuated from a first to a second condition upon a need for operation of said fuel burner unit; time delayed safety cutout means having an electrically operable actuator and a normally closed switch actuated to open position a time period after energization of said actuator; means for energizing said control means actuator and the actuator of said safety cutout means upon a need for operation of said fuel burner unit, holding means to maintain said control means actuator energized and including the normally closed switch of said safety cutout means; means controlled by said control means switch means in said second condition for de-energizing the actuator of said safety cutout means; circuit means completed by said control means switch means in said second condition for energizing said blower; prepurge timing means having an electrically operable actuator and switch means for energizing said pilot burner valve and said ignition means and for re-energizing the actuator of said safety cutout means a time period after energization of said blower; flame detecting means for detecting flame at said pilot and main burners and for energizing said main burner valve upon flame being established at said pilot burner; ignition timing means for de-energizing said ignition means a time period after said pilot burner valve and said ignition means are energized; and means controlled by said flame detection means and said ignition timing means to render the flame detecting means inoperative to detect flame if flame fails after said ignition means is de-energized.

15. Control apparatus for use with a fuel burner unit having electrical ignition means, an electrically energizable main burner and pilot burner, and having a blower motor, comprising: a first electron discharge device, a first relay having switch means and having a winding connected in circuit with said first discharge device so that said first relay winding is energized upon said first electron discharge device conducting current, a second electron discharge device having a control electrode, means connecting said second discharge device in controlling relation to said first discharge device to render said first discharge device nonconductive in the absence of a signal at the control electrode of said second discharge device, flame sensing means arranged to sense flame at the pilot and main burners, circuit means connecting said flame sensing means to the control electrode of said second discharge device to apply a signal to said electrode upon the presence of flame at the burners,

power input terminals, means rendering only said first discharge device operative upon initial application of power to said terminals to thereby cause said first relay winding to be energized, a second relay having a winding and switch means, circuit means controlled by said first relay switch means to energize said second relay winding upon energization of said first relay winding, a holding circuit for said second relay winding, means controlled by said second relay switch means when said second relay winding is energized to render said second discharge device operative to thereby cause said first relay winding to be de-energized, a time delayed safety cutout device having an electrically operable actuator and a normally closed switch, a third relay having a winding and switch means, energizing circuit means for said third relay winding including the actuator of said safety cutout device and controlled by said first and second relay switch means and completed when said first relay winding is de-energized and said second relay winding is energized, holding circuit means for said third relay winding including the normally closed switch of said safety cutout device, means for de-energizing said second relay winding to thereby open said energizing circuit means upon energization of said third relay winding; means controlled by said third relay switch means for energizing the blower motor upon energization of said third relay winding, prepurge timer means for energizing the pilot burner and ignition means a given time interval after the blower motor is energized, means controlled by said first relay switch means for energizing the main burner upon flame being established at the pilot burner, ignition timing means to de-energize the ignition means a given time interval after the ignition means is initially energized, and means controlled by said ignition timing means connected to the control electrode of said second discharge device to render said second discharge device incapable of responding to a signal indicative of flame if a flame failure occurs after the ignition means is de-energized.

References Cited in the file of this patent UNITED STATES PATENTS 2,170,497 Gille Aug. 22, 1939 2,313,943 Jones Mar. 16, 1943 2,537,293 Peterson Jan. 9, 1951 2,616,490 Wilson Nov. 4, 1952 2,655,208 Outterson Oct. 13, 1953 2,695,661 Porter Nov. 30, 1954 

